1. Field of the Invention
The present invention relates to a semiconductor laser control system for driving and controlling a semiconductor laser which is used as a light source in each of a laser printer, a digital copier, an optical disc drive device, an optical communications apparatus and so forth.
2. Description of Related Art
A semiconductor laser is very small, and it is possible to modulate a light output at high speed through a driving current. Therefore, a semiconductor laser is widely used recently as a light source of a laser printer or the like.
However, because the relationship between a driving current and a light output of a semiconductor laser varies greatly due to temperature change, it may be difficult to precisely set the light output of a semiconductor laser to a desired value. In order to solve this problem and to effectively use the advantages of a semiconductor laser, various APC (Automatic Power Control) electric circuits have been proposed.
The APC electric circuits can be broadly divided into the following three systems:
In a first system, the light output of a semiconductor laser is monitored through a light reception device. A negative feedback loop always controls the forward current of the semiconductor laser so that a signal in proportion to a light reception current which occurs in the light reception device and is in proportion to the light output of the semiconductor laser may be equal to a light emission instruction signal. Thus, the light output of the semiconductor laser may be controlled to be a desired value. (Throuout the present application, the `negative feedback loop` includes electricity-to-light conversion at a portion where a semiconductor laser emits light and light-to-electricity conversion at a portion where a light reception device receives the light emitted by the semiconductor laser.)
In a second system, during a power setting period, the light output of a semiconductor laser is monitored through a light reception device; and a negative feedback loop always controls the forward current of the semiconductor laser so that a signal in proportion to a light reception current which occurs in the light reception device and is in proportion to the light output of the semiconductor laser may be equal to a light emission instruction signal. During the time other than the power setting period, the forward current which has been set during the power setting period is maintained, and thus, the light output of the semiconductor laser is controlled to a desired value. Further, during a time other than the power setting period, the forward current which has been set during the power setting period is modulated based on information, and thereby, the information is carried on the light output of the semiconductor laser.
In a third system, the temperature of a semiconductor laser is measured, and thus, a temperature signal is obtained. Through the temperature signal, the forward current of the semiconductor laser is controlled, or the temperature of the semiconductor is controlled to be fixed. Thereby, the light output of the semiconductor laser is controlled to a desired value.
In order to obtain a desired value of light output, the first system is preferable. However, in the first system, a control speed has a limit due to the limits of the operation speed of the light reception device, the operation speed of an amplifying device which is used in the negative feedback loop and so forth. For example, when the cutoff frequency f.sub.0 of the negative feedback loop in the open loop condition is considered, the step response characteristics of the semiconductor laser can be approximated, as follows: EQU P.sub.out =P.sub.0 {1-exp(-2.pi.f.sub.0 t)},
where:
P.sub.out represents light output of the semiconductor laser; PA1 P.sub.0 represents a set light intensity of the semiconductor laser; and PA1 t: represents time. PA1 a light intensity modulation (power modulation) method; PA1 a pulse width modulation method; and PA1 a pulse width and intensity combined modulation method. PA1 a pulse width modulation and intensity modulation signal generating unit which, based on input data, performs pulse width modulation and intensity modulation and thus generates a light emission instruction signal; PA1 an error amplifier which forms a negative feedback loop with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controlling forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal; and PA1 a current driving unit which provides a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current thereof, the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop, PA1 wherein the pulse width modulation and intensity modulation signal generating unit, the error amplifier and the current driving unit are formed to be one chip of an integrated circuit. PA1 data converting means for converting the input data into pulse modulation data and intensity modulation data; PA1 pulse width modulation means which, based on the pulse modulation data, generates a plurality of pulse-modulated pulses; and PA1 a light emission instruction signal generating unit which, based on the outputs of the data converting means and the pulse width modulation means, performs the pulse width modulation and intensity modulation and generates the light emission instruction signal for the semiconductor laser. PA1 pulse width modulation and intensity modulation generating unit which, based on input data, performs pulse width modulation and intensity modulation and generates a light emission instruction signal; PA1 an error amplifier which forms a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controlling forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal; and PA1 a current driving unit providing a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current, the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop, PA1 wherein: PA1 a pulse width modulation and intensity modulation signal generating unit which, based on input data, performs pulse width modulation and intensity modulation and generates a light emission instruction signal; PA1 an error amplifier forms a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controlling forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal; and PA1 a current driving unit for causing a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current, the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop, PA1 a differential quantum efficiency detecting unit for detecting the differential quantum efficiency of the semiconductor laser; PA1 a memory unit for storing a detection result of the differential quantum efficiency detecting unit; PA1 an adding current setting unit for setting a current, corresponding to the light emission instruction signal, using the detection result stored in the memory unit; and PA1 a timing generating unit, PA1 wherein, in initialization, the timing generating units generates a timing signal sufficiently slower than the control speed of the error amplifier, the differential quantum efficiency detecting unit detects the differential quantum efficiency of the semiconductor laser based on the timing signal, the memory units stores a detection result at each timing, and the current corresponding to the light emission instruction signal is set using the stored detection results. PA1 a pulse width modulation and intensity modulation signal generating unit which, based on input data, performs pulse width modulation and intensity modulation and generates a light emission instruction signal; PA1 an error amplifier forms a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controlling forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal; and PA1 a current driving unit providing a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current, the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop, PA1 a differential quantum efficiency detecting unit for detecting the differential quantum efficiency of the semiconductor laser; PA1 a timing generating unit for generating a timing signal which controls a detection operation of the differential quantum efficiency detecting unit in initialization; PA1 a memory unit for storing a detection result of the differential quantum efficiency detecting unit at each timing; and PA1 an adding current setting unit for setting a current, corresponding to the light emission instruction signal, using the detection results stored by the memory unit. PA1 the current driving unit comprises a voltage shifting unit in the error amplifier, includes a differential circuit for changing the amount of voltage shift, and is provided in the negative feedback loop; PA1 the adding current setting unit sets a current of the differential circuit so that light output of the semiconductor laser is a desired maximum value when a current corresponding to the light emission instruction signal is maximum, and light output of the semiconductor laser is a desired minimum value when a current corresponding to the light emission instruction signal is minimum; PA1 in initialization, light output of the semiconductor laser is set to the desired maximum value at a certain timing T0, light output of the semiconductor laser is set to the desired minimum value at a timing T1 after a fixed time has elapsed from the timing T0, the differential quantum efficiency detecting unit and the adding current setting unit are operated and the current is set between the timing T1 and a timing T2 after a fixed time has elapsed from the timing T1. PA1 a pulse width modulation and intensity modulation signal generating unit which, based on input data, performs pulse width modulation and intensity modulation and generates a light emission instruction signal; PA1 an error amplifier which forms a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controlling forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal; and PA1 a current driving unit providing a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current, the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop, PA1 a differential quantum efficiency detecting unit for detecting the differential quantum efficiency of the semiconductor laser; PA1 a memory unit for storing a detection result of the differential quantum efficiency detecting unit; PA1 an adding current setting unit for setting a current, corresponding to the light emission instruction signal, using the detection result stored in the memory unit; PA1 a timing generating unit; and PA1 a switch unit, to which a forcible light emission instruction signal and a forcible light cessation instruction signal are selectively input, the switch unit providing an output selected from outputs including the light emission instruction signal based on input data; PA1 wherein: PA1 a pulse width modulation and intensity modulation signal generating unit which, based on input data, performs pulse width modulation and intensity modulation and generates a light emission instruction signal; PA1 an error amplifier forms a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controlling forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal; and PA1 a current driving unit which causes a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current, the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop, PA1 a switch unit, to which a forcible light emission instruction signal and a forcible light cessation instruction signal are selectively input, the switch unit providing an output selected from outputs including the light emission instruction signal based on input data; PA1 a differential quantum efficiency detecting unit for detecting the differential quantum efficiency of the semiconductor laser based on a timing signal; PA1 a timing generating unit for generating a timing signal which is sufficiently slower than the control speed of the error amplifier, for controlling a detection operation of the differential quantum efficiency detecting unit, in initialization; PA1 a memory unit for storing a detection result of the differential quantum efficiency detecting unit at each timing; and PA1 an adding current setting unit for setting a current, corresponding to the light emission instruction signal or the forcible light emission instruction signal, using the detection results stored by the memory unit. PA1 a pulse generating means for generating a plurality of pulses having a frequency the same as the frequency of an input clock signal and having different phases, the phase difference being a fixed phase difference; PA1 data converting means for converting input data into pulse width modulation data and power modulation data; and PA1 pulse width modulation means for generating a plurality of pulses which have undergone pulse width modulation, based on the pulse width modulation data, from the pulses generated by the pulse generating means. PA1 a pulse width modulation and intensity modulation signal generating unit which, based on input data, performs pulse width modulation and intensity modulation and generates a light emission instruction signal; PA1 an error amplifier which forms a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controlling forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal; and PA1 a current driving unit which causes a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current, the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop, PA1 wherein: PA1 a combined setting unit may be provided having an external connection device for combined setting of a maximum current of the pulse width modulation and intensity modulation signal generating unit and the offset current. PA1 a light emission instruction signal generating unit which causes an absolute current determined by the light reception device to flow, may have a base current compensation unit for compensating base currents of transistors connected to a path of a reference current. PA1 a pulse width modulation and intensity modulation signal generating unit which comprises data converting means for converting the input data into pulse modulation data and intensity modulation data, pulse width modulation means which, based on the pulse modulation data, generates a plurality of pulse-modulated pulses, and a light emission instruction signal generating unit which, based on the outputs of the data converting means and the pulse width modulation means, performs the pulse width modulation and intensity modulation and generates a light emission instruction signal for the semiconductor laser, the pulse width modulation and intensity modulation signal generating unit, based on input data, performing pulse width modulation and intensity modulation and generates the light emission instruction signal; PA1 an error amplifier which forms a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controlling forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal; and PA1 a current driving unit providing a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current, the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop, PA1 wherein: PA1 an external connection device may be provided for setting the control speed of the negative feedback loop. PA1 a pulse width modulation and intensity modulation signal generating unit which, based on input data, performs pulse width modulation and intensity modulation and generates a light emission instruction signal; PA1 an error amplifier forming a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controls forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal; PA1 a current driving unit providing a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current , the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop; and PA1 output mode change-over means for selecting one of clock-frequency-different output modes according to a frequency selecting signal. PA1 a pulse width modulation and intensity modulation signal generating unit which, based on input data, performs pulse width modulation and intensity modulation and generates a light emission instruction signal; PA1 an error amplifier which forms a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controlling forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal; PA1 a current driving unit providing a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current, the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop; and PA1 output mode change-over means for selecting one of different clock-frequency output modes according to a frequency selecting signal, PA1 wherein the pulse width modulation and intensity modulation signal generating unit, the error amplifier, the current driving unit and the output mode change-over means are formed as one chip of an integrated circuit. PA1 a pulse width modulation and intensity modulation signal generating unit comprising pulse generating means, including an pulse oscillator, for generating a plurality of pulses having a frequency the same as the frequency of an input clock signal and having different phases, the phase difference being a fixed phase difference, data converting means for converting input image data into pulse width modulation data and intensity modulation data and pulse width modulation means for generating a plurality of pulses, which have undergone pulse width modulation based on the pulse width modulation data, from the pulses generated by said pulse generating means, said pulse width modulation and intensity modulation signal generating unit performing pulse width modulation and intensity modulation and generating a light emission instruction signal; PA1 an error amplifier which forms a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controlling forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal; PA1 a current driving unit providing a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current, the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop; and PA1 wherein the pulse width modulation and intensity modulation signal generating unit, the error amplifier and the current driving unit are formed as one chip of an integrated circuit. PA1 a pulse width modulation and intensity modulation signal generating unit comprising pulse generating means for generating a plurality of pulses having a frequency the same as the frequency of an input clock signal and having different phases, the phase difference being a fixed phase difference, a fixed amount by the fixed amount, data converting means for converting input image data into pulse width modulation data and intensity modulation data and pulse width modulation means for generating a plurality of pulses, which have undergone pulse width modulation based on the pulse width modulation data, from the pulses generated by the pulse generating means, the pulse width modulation and intensity modulation signal generating unit performing pulse width modulation and intensity modulation and generating a light emission instruction signal; PA1 an error amplifier providing a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of the semiconductor laser, the error amplifier controlling forward current of the semiconductor laser so that a light reception signal proportional to the light output of the semiconductor laser is equal to the light emission instruction signal; and PA1 a current driving unit providing a driving current, according to the light emission instruction signal, to flow through the semiconductor laser as the forward current, the driving current being generated so as to control driving of the semiconductor laser with a current of the difference or sum with the control current of the negative feedback loop, PA1 wherein: PA1 a pulse width modulation and intensity modulation signal generating unit which, based on input data, performs pulse width modulation and intensity modulation and generates a light emission instruction signal; PA1 an error amplifier providing a negative feedback loop together with a semiconductor laser and a light reception device which monitors light output of said semiconductor laser, said error amplifier controlling forward current of said semiconductor laser so that a light reception signal proportional to the light output of said semiconductor laser is equal to the light emission instruction signal; and PA1 a current driving unit providing a driving current, according to the light emission instruction signal, to flow through said semiconductor laser as the forward current, the driving current being generated so as to control driving of said semiconductor laser with a current of the difference or sum with the control current of said negative feedback loop, PA1 a differential quantum efficiency detecting unit for detecting the differential quantum efficiency of said semiconductor laser; PA1 a memory unit for storing a detection result of said differential quantum efficiency detecting unit; PA1 an adding current setting unit for setting a current, corresponding to the light emission instruction signal, using the detection result stored in said memory unit; and PA1 a timing generating unit, PA1 wherein:
In many cases, it is required that the total light quantity (the integral value of the light output: .intg.P.sub.out .multidot.dt) until a set time .tau..sub.0 has passed immediately after a light intensity of the semiconductor laser was changed should be a predetermined value, where: EQU .intg.P.sub.out .multidot.dt=P.sub.0 .multidot..tau..sub.0 {1-1/(2.pi.f.sub.0 .tau..sub.0)!1-exp(-2.pi.f.sub.0 .tau..sub.0)!}.
Assuming that .tau..sub.0 =50 (ns) and an error allowance is 0.4%, it should be that f.sub.0 &gt;800 (MHz), and it is very difficult to satisfy this condition.
In the second system, the above-described problem occurring in the first system does not occur, and thus, it is possible to modulate the light output of the semiconductor laser at high speed. Therefore, the second system is widely used. However, according to the second system, the light output of the semiconductor laser is not always controlled. Therefore, an external disturbance or the like may easily cause the light intensity of the semiconductor laser to vary. For example, the Do loop characteristics of a semiconductor laser may easily cause the light intensity of the semiconductor laser to include an error of several percent. As an attempt to restrict the Do loop characteristics of a semiconductor laser, a method has been proposed in which the heat time constant of the semiconductor laser is matched by the frequency characteristics of a semiconductor laser driving current and thus the Do loop characteristics are compensated. However, the heat time constant of a semiconductor laser varies among respective particular semiconductor lasers, and also, it varies due to an ambient condition. Therefore, such a method may not be effective.
For example, the applicant of the present invention proposed an improvement in consideration of such a problem in Japanese Laid-Open Patent Application No.2-205086 (corresponding U.S. Pat. No. 5,036,519). According to the proposed method, as shown in FIG. 1, a light reception device 2 monitors the light output of a semiconductor laser 1. A negative feedback loop 3 always controls the forward current of the semiconductor laser 1 so that an output signal of the light reception device 2 may be equal to a light emission instruction signal (DATA). A current driving unit 4 converts the light emission instruction signal (DATA) into the forward current of the semiconductor laser 1. The light output of the semiconductor laser 1 is controlled through the current which is the sum (or difference) of a control current of the negative feedback loop 3 and a driving current generated by the current driving unit 4. In the example shown in FIG. 1, the negative feedback loop 3 includes the semiconductor laser 1, the light reception device 2, a constant-current source 5 of a constant current I.sub.DA1 and an inverting amplifier 6. The output of the inverting amplifier 6 is used to drive and control a driving transistor 7. The semiconductor laser 1, the driving transistor 7 and a resistor Re are connected in series as shown in the figure. The current driving unit 4 includes a constant-current source 8 of a constant current I.sub.DA2.
In the circuit configuration, when light output corresponding to a current through which the current driving unit 4 directly drives the semiconductor laser 1 is referred to as P.sub.S, the step response characteristics of the light output of the semiconductor laser can be approximated as follows : EQU P.sub.out =P.sub.0 +(P.sub.S -P.sub.0){1-exp(-2.pi.f.sub.0 t)}.
When P.sub.S .apprxeq.P.sub.0), the light output of the semiconductor laser 1 immediately becomes equal to P.sub.0. Therefore, f.sub.0 may have a relatively small value in comparison to the case where there is only the negative feedback loop 3. FIG. 2A shows how the light output changes only through the negative feedback loop 3 (control unit). FIG. 2B shows how the light output changes in the case where the constant current I.sub.DA2 is added by the current driving unit 4. In a practical case, f.sub.0 may have a value of approximately 40 (MHz). Such an amount of cutoff frequency f.sub.0 can be easily obtained.
The applicant of the present invention also disclosed a semiconductor laser control system in Japanese Laid-Open Patent Application No.5-67833 (corresponding U.S. Pat. No. 5,237,579). In the disclosed system, bipolar transistors are used as elements of the configuration of Japanese Laid-Open Patent Application No.2-205086 described above, and thus an IC is formed. Thereby, it is easy to design a negative feedback loop.
A one-dot multi-level method will now be described, in a case where a laser printer is taken as an example. A laser printer was developed as a non-impact printer for taking the place of a line printer. Because of a high speed printing characteristic and a high resolution characteristic of the laser printer, application of the laser printer to an image printer was attempted. As a result, various printing methods, which use a dither method as basic technology, have comes to be practically used in laser printers. Further, as a result of recent quick development of semiconductor technology, the amount of information which can be processed by a laser printer has quickly increased. AS a result, in a laser printer, a one-dot multi-level method is practically used, and thereby, a laser printer is effectively used as an image printer. In the one-dot multi-level method, in a high-end machine, the number of tone levels is that which can be obtained in the use of 8 bits. However, in a low-end machine, the number of tone levels is a low number (several levels). One reason is that the information amount to be processed increases when the number of tone levels increases. However, a main reason is that the scale and costs of the electrical circuit of a semiconductor laser control modulation unit increases when the number of tone levels increases.
Currently, the following three semiconductor laser control modulation methods have been proposed:
The light intensity modulation (power modulation, which may be abbreviated to `PM`) method will now be described. In the method, light output itself is changed when a dot is printed. In this method, a middle exposure range is used for obtaining middle tone levels. Therefore, it is important that a printing process should be stabilized, and thus requirements for the printing process are strict. However, in this method, semiconductor laser control modulation can be easily performed.
The pulse width modulation (which may be abbreviated to `PWM`) method will now be described. In this method, there are two light output levels. A time of light emission is changed (that is, a pulse width of a light emission instruction signal is changed) when a dot is printed. Therefore, in comparison to the PM method, the middle exposure range is less used. Further, by coupling adjacent dots, it is possible to further decrease use of the middle exposure range. As a result, requirements for printing process stability are reduced. However, when pulse width setting is performed with 8-bit data and coupling of adjacent dots is performed, the configuration of a semiconductor laser control modulation unit is complex.
The pulse width and intensity combined modulation method (PWM+PM method) will now be described. As mentioned above, requirements for a printing process are strict in the PM method and a semiconductor laser control modulation unit is complex in the PWM method. In order to solve the problems, the pulse width and intensity combined method was considered. For example, the applicant of the present invention discloses this method in Japanese Laid-Open Patent Application No.6-347852 (corresponding U.S. patent application Ser. No. 08/253,322), now abandoned and refiled as U.S. Ser. No. 08/862,326.
This modulation method is basically a two-level printing method, and thus is a method using the PWM method, in which requirements for printing process stability are not strict, as a basic technology. The PM method is used for interpolate a change of a pulse width. When the same resolution is obtained, in comparison to a case of each separate modulation method (the PM and PWM methods), each of the number of pulse widths and the number of power levels can be reduced. This is because, in the combined modulation method, the resolution is provided as a result of combining the number of pulse widths and the number of power levels. As a result, it is possible to easily provide an arrangement required for each separate modulation method. Thereby, requirements for printing process stability are not strict, and also it is suitable to provide an integrated circuit which performs the combined modulation method. Thus, it is possible to miniaturize and reduce costs of an arrangement required for performing the combined modulation method.
FIG. 3 shows an example of an arrangement of a semiconductor laser control system which performs the combined modulation method. In the arrangement, image data and an input clock signal are input to a pulse width generating unit and data modulation unit 11. The pulse width generating unit and data modulation unit 11 outputs a light emission instruction signal (DATA) to a semiconductor laser control unit and semiconductor laser driving unit 12. The semiconductor laser control unit and semiconductor laser driving unit 12 has, for example, a circuit configuration such as that shown in FIG. 1. According to the input image data, the pulse width generating unit and data modulation unit 11 performs basically the PWM method, and the PM method for interpolating a change of a pulse width.
A basic concept of light output waveforms of the semiconductor laser 1 is shown in FIG. 4. For the sake of simplicity of description, in the example shown in FIG. 4, there are three pulse widths and six power levels. Thereby, a total of 18 tone levels are output. FIG. 4 typically shows light output waveforms in this case. As shown in the figure, in this combined modulation method, basically the PWM method is used. Power modulation using a middle exposure range is performed on a minimum pulse width. In order to obtain those light output waveforms, for example, as shown in FIGS. 5A and 5B, either a pulse 1 having a pulse width of T and a pulse 2 having a pulse width of T+.delta.T (.delta.T being the minimum pulse width) or a pulse 3 having a pulse width of T and a pulse 4 having a pulse width of .delta.T are generated. For the pulse of the pulse width of T, each of the bits is of the H level, while, for the pulse of the pulse width .delta.T (in the case of the pulse 2, during the time .delta.T only the pulse 2 is at the high level), respective bits may be of either the H level or the L level according to the PM data. Thereby, the light output waveforms shown in FIG. 4 and FIGS. 5A, 5B can be provided. In the example shown in FIG. 5A, a pulse width starts from the left side, and, in the example shown in FIG. 5B, a pulse width starts from the right side.
For example, Japanese Laid-Open Patent Application No.6-347852 discloses that, for implementing such a pulse width and intensity combined modulation system within one dot, a pulse width generating unit is formed as an IC using C-MOS devices and thus the unit is easily provided, and also, a negative feedback loop unit is formed as an IC using bipolar transistors and is easily designed.
However, methods disclosed in Japanese Laid-Open Patent Application No.6-347852 may be further improved. It is considered that a current adding method by which a control amount of the negative feedback loop can be effectively reduced and a pulse width and intensity combined modulation method in a pulse (pulses) within one dot may be implemented by a further miniaturized and increased power saving configuration, and also may function at higher speed and with higher accuracy.
When harnesses or the like are used for data transfer, there are problems which will be described below. For example, in a case, disclosed in Japanese Laid-Open Patent Application No.6-347852, where a pulse width generating unit and a data modulation unit, to which input data is input, are formed as C-MOS devices, because the operational principle of C-MOS devices is generally a switching operation (that is, an turning on-turning off operation), it is necessary that the input level of the input data should be 0 to 5 volts or 0 to 3 volts. In a transmission path such as a harness or the like, characteristic impedance Z of the line is expressed as: EQU Z=(L/C).sup.1/2.
Assuming that L=10 (mH/cm) and C=0.7 (pF/cm), Z.apprxeq.120 (.OMEGA.). FIG. 6 shows an example of input data using a harness 15 in a case of an integrated circuit 14 using a C-MOS device. For example, input data is input to the integrated circuit 14 via a connecting point at which voltage dividing resistors R.sub.A (330 .OMEGA.) and R.sub.B (220 .OMEGA.) are connected with one another. In the integrated circuit 14, a constant-current source 16 is included as shown in the figure. Assuming that the length of the harness 15 is 50 cm, a rising time constant .tau. of a data waveform of input data during data transfer through the harness 15 is expressed as: EQU .tau.=CR=35(pF).multidot.120(.OMEGA.)=4(ns).
Considering 10 to 90% of the full variation range of the data, as shown in FIG. 7, EQU t.apprxeq.2.5.tau.=10(ns).
Thereby, when data transfer is performed in a synchronization condition, it is possible to perform data transfer at 25 (MHz) (corresponding to 40 ns) at the highest, and it is difficult to perform data transfer at a higher speed than 25 (MHz). When the pulse width generating unit and data modulation unit are formed simply with bipolar transistors, generally speaking, an input level of input data is set to 0 to 5 volts or 0 to 3 volts in consideration of an interface in a case where such an electronic circuit may be connected with a C-MOS device. Therefore, the same problem may occur, and it is difficult to perform data transfer at higher speed. Further, because the amount of voltage swing of the input is large as mentioned above, taking EMI (Electro-Magnetic Interference) measures and achieving power saving may be difficult.
Further, in such a semiconductor laser control system, a current in a light emission instruction signal generating unit is, when considering only a direct current component, a monitoring current of a monitoring light reception device. Therefore, when the electronic circuit is formed as an integrated circuit, it is necessary that the current in the light emission instruction signal should be a current which does not vary depending on changes of the integrated circuit internal temperature. When no special measures are taken therefor, stabilization of the monitoring current may cause problems. When stabilization of the monitoring current is not ensured, the semiconductor laser control system cannot perform an adequate operation for a wide range of the monitoring current.
Further, when a control speed is uniformly set for a control system of the negative feedback loop, flexibility in designing the control system may be reduced and it is not possible to freely set the control speed to a desired speed.
For the purpose of obtaining, under high-speed control, an ideal waveform of light output which is always optimized, it is important to appropriately set the level P.sub.S shown in FIG. 2B, and thereby to cause the light output waveform to approximate that of a rectangular wave. This is important, in particular, in a case where modulation is performed for a larger number of tone levels using pulse width and intensity combined modulation method which was described with reference to FIGS. 4, 5A and 5B.
As general characteristics of a semiconductor laser, there is a current change due to a temperature change shown in FIG. 8, and also, there is a current change due to an elapsing time change (especially, a change of a differential quantum efficiency) shown in FIG. 9. For the characteristic of an operation current change due to temperature change, a semiconductor laser control system can appropriately respond thereto by causing a negative feedback loop 3 such as that shown in FIG. 1 to always operate, whereby, even when the oscillation threshold current Ith of the semiconductor laser varies, the control system follows it. Thereby, the control system causes the oscillation threshold current Ith to flow through the semiconductor laser 1 as its forward current.
However, as shown in FIG. 9, when there is a change of an operation current due to elapsing time, especially due to a differential quantum efficiency, the change in an operation current is larger than that in an operation current due to a temperature change. Such change characteristics due to a differential quantum efficiency affects the P.sub.S level shown in FIG. 2B. Thereby, actual light output is too high in comparison to a desired light output P.sub.out, and thus an overshoot occurs, or actual light output is too low in comparison to a desired light output P.sub.out, and thus an undershoot occurs. Thus, high-speed control cannot be achieved.
In the related art, a problem may be the lack of accuracy in detection of a differential quantum efficiency of a semiconductor laser, and adaptability thereto may be insufficient. Thereby, it may be difficult to provide a light output waveform which approximates that of a rectangular wave.